1. Field of the Invention
This invention relates to magnetic sensing and reading devices and, more particularly, is concerned with detectors (or readers) for reading information stored in magnetic tags or elements.
2. Description of the Related Art
In previous patent applications (GB 9506909.2 and PCT/GB96/00823xe2x80x94published as WO96/31790xe2x80x94and PCT/GB97/01662xe2x80x94published as WO97/48990) we have described novel techniques for spatial magnetic interrogation based on exploiting the behaviour of magnetic materials as they pass through a region of space containing a magnetic null. The disclosures of these documents are incorporated herein by way of reference thereto. In particular, our earlier applications describe how passive tags containing one or more magnetic elements can perform as remotely-readable data carriers, the number and spatial of the elements representing information.
A fundamental aspect of the present invention relies upon the interaction between a magnetically active element, and the magnetic field to which it is subjected. As is well defined in the afore mentioned earlier application, particular use can be made of magnetic fields which contain a xe2x80x9cmagnetic nullxe2x80x9d. This term is used herein to mean a point, line, plane or volume in space at or within which the component of the magnetic field in a given linear direction is zero.
According to the present invention, there is provided a magnetic reading device for use in reading magnetically coded information carriers including elements having an easy axis of magnetisation, which comprises (1) a plurality of permanent magnets consisting of two assemblies of magnets, wherein said assemblies are disposed so as to face one another, thereby defining a gap through which said magnetically coded information carrier is, in use, passed, (2) receiver coils comprising one or more pairs of coils connected in antiphase, said coils being positioned within said gap, characterised in that:
(i) the two assemblies oppose one another magnetically, in that those magnets which directly face each other, are of the same polarity;
(ii) the magnetic field pattern generated by said plurality of magnets is such that the magnetic polarity or said magnetically coded information carrier reverses as it passes through said magnetic field with the easy axes of magnetisation of the magnetic elements oriented in the direction of travel;
(iii) said receiver coils are arranged so as to detect magnetic dipole radiation emitted by the magnetic material in the course of its passage through the magnetic field.
When the magnetic material crosses through the magnetic field generated by a magnetic reading device of the present invention the magnetic polarity of the material reverses. Magnetic dipole radiation is emitted by the material during this reversal which may then be detected by receiver coils.
In a preferred embodiment of the present invention, the detector comprises two triplets of permanent magnets arranged in magnetic opposition, the space between the magnets defining the spatial region which is in the form of a slot through which the tag, when in use, is passed. The central magnet of each of the triple magnet assemblies is the main field-generating magnet; the magnets which flank the central magnet are subsidiary magnets which serve to modify the overall magnetic field in an advantageous manner.
According to a second aspect of the present invention, there is provided a detector for sensing the presence of a magnetic tag having an axis of easy magnetisation, which comprises (1) an arrangement of six permanent magnets disposed so as to define a gap through which the magnetic tag is, in use, passed, the disposition of said magnet(s) and the resultant magnetic field pattern being such as to cause a change in polarity of the magnetisation of said magnetic tag in the course of its passage through a magnetic null within said spatial region; and (2) receiver coils comprising one or more pair(s) of coils connected in antiphase arrangement, the coils being arranged to detect magnetic dipole radiation emitted by a magnetic tag as it passes through said magnetic null with the easy axis of magnetisation of the tag oriented in the direction of travel, characterised in that:
(1) the arrangement of magnets consists of two assemblies of magnets each of which comprises three elongate magnets placed side by side;
(2) the major axes of all six magnets are mutually parallel;
(3) the two assemblies oppose one another physically, thereby defining said gap through which, in use, the magnetic tag is passed;
(4) the two assemblies oppose one another magnetically in that the physically opposed magnets from each assembly are arranged such that their like poles are directed towards said gap; and
(5) said receiver coils are positioned within said gap.
Advantageously, the receiver coils are positioned on the inwardly directed face of the central magnet of each of said assemblies. To provide protection from unwanted environmental influences, the coils are preferably covered by an electrostatic screening layer.
In one embodiment, there are four balanced coils open along an axis perpendicular to the direction of movement of a magnetic tag through said gap.
Preferably, the detector is mounted in a housing which surrounds said two assemblies and provides electrical screening therefor, said housing having a slot which permits access to the gap between the two assemblies of magnets.
As mentioned above, the receiver coil arrangement can consist of 4 balanced coils, open along an axis normal to the tag trajectory. This arrangement of magnets and receiver coils is particularly suited to the non-contact reading of tags containing material of medium or high coercivity or where the field required for saturation (due to shape factors for example) is high. The invention is particularly relevant to the detection of discrete elements made from medium coercivity magnetic media, of the kind commonly used for security purposes in documents such as banknotes and cheques.